Are traditional mass media outlets important in determining what issues voters care about?
Traditional mass media outlets are important, but they are by no means the most important determinant of what issues are salient to people.
First, we must understand that there are other sources than the media that expose people to issues that may become important to them. For example, people hear about issues from their friends and family and people at work. They hear about issues from people at church and from non-mass media sources such as bloggers.
Second, we must understand that simply being exposed to an issue does not make people think it is important. A person might read about income inequality a great deal, for example, without feeling that it is important to their lives. The same goes for issues like abortion or gay marriage. It is the person's own philosophy (which comes largely from sources other than the media) that determines which of the many issues become salient.
Thus, the mass media is by no means the most important determinant of what we feel is important. However, it does play some role in this determination.